1. Field of the Invention
The present invention relates to a superconducting device which operates at the liquid nitrogen temperature or above, and more particularly to a superconducting device which is readily produced and which operates stably.
2. Description of the Related Art
Heretofore, materials such as Nb.sub.3 Ge have been used as the materials of superconducting devices which operate at high temperatures. This technique is discussed by H. Rogalla et al. in "IEEE Transactions," MAG-15, 536 (1985).
A prior-art superconducting device in which a plurality of electrodes exhibitive of a superconductivity are coupled through a semiconductor or a normal-conductor, is discussed by R. B. van Dover et al. in "Journal of Applied Physics," vol. 52, p. 7327, 1981. Besides, a three-terminal superconducting device in which the above superconducting device is additionally provided with means for changing the coupling between the superconducting electrodes on the basis of the field effect is discussed by T. D. Clark et al. in "Journal of Applied Physics," vol. 5, p. 2736, 1980. The sectional structure of the three-terminal superconducting device is shown in FIG. 1. In this device, the value of a superconducting current to flow via a semiconductor layer 2 across two superconducting electrodes 3a and 3b disposed in contact with the semiconductor layer 2 on a substrate 1 is controlled in such a way that the superconducting proximity effect is changed by a voltage applied to a control electrode 5 disposed between both the electrodes 3a and 3b. The control electrode 5 is disposed on the semiconductor layer 2 through an electric insulator film 4.
The prior art has used Pb, Pb alloys, Nb and Nb compounds as the materials of the superconducting electrodes. In order to operate the superconducting device employing any of these materials, accordingly, the device must be installed in the atmosphere of a cryogenic temperature near the liquid helium temperature (4.2 K.). Further, the two superconducting electrodes must be provided at a spacing within 0.5 .mu.m in order to intensify the influence of the superconducting proximity effect between these superconducting electrodes, and this has made the fabrication of the device very difficult.
Moreover, in the prior art, the superconducting electrodes and the semiconductor or normal-conductor have been made of different materials of elements. By way of example, the material of the superconducting electrodes has been any of Nb, Pb alloys, Sn etc., while the material of the semiconductor or normal-conductor has been any of Si, InAs, Cu, etc. The combination of these materials, however, signifies that the device is constructed by stacking the materials of the superconductors and the semiconductor or normal-conductor, the electrical properties of which are quite different. That is, the superconducting device has a structure in which the surface of the semiconductor or normal-conductor is overlaid with the superconductors made of the different material. On this occasion, the characteristics of the superconductors are highly susceptible to the state of the surface of the semiconductor or normal-conductor, so that the characteristics of the device of such a structure are liable to change. It has therefore been difficult to reproducibly fabricate the superconducting device of this type.
The superconducting critical temperature (T.sub.c) of the superconductors is at most 10-20 K. or so. This signifies that the characteristics of the device are prone to become unstable due to the temperature change thereof.
Since the prior-art superconducting device operates chiefly at the liquid helium temperature, it has been cooled down to that temperature by a method of immersion in liquid helium or cooling with helium gas. The liquid helium, however, is very expensive and is uneconomical as a coolant. Another problem has been that, since the liquid helium is at the temperature much lower than the room temperature, the handling thereof is, in itself, difficult. These problems of the liquid helium have directly led to problems on the economy and handling of the superconducting device itself.
In addition, the superconducting materials having heretofore been used are polycrystalline or amorphous. With the polycrystalline material, it is difficult to precisely microfabricate a part of or less than 0.5 .mu.m. Besides, in case of using a material whose property as a superconductor depends upon the orientation of a crystal, the degree of the crystal orientation of the crystal grain of the polycrystalline material needs to be strictly controlled each time the superconductor is fabricated. In general, however, this control is difficult and has therefore incurred the problem that variation in characteristics at the stage of manufacture becomes large.
Typical as the structure of a prior-art superconducting device having a superconducting weak-link element is the so-called "micro-bridge" in which a superconducting film is partially fined to form a constriction, the constricted portion being endowed with a weak link property. Especially for the Nb-type superconducting material, optical patterning technology or electron-beam lithography and technology for processing the superconducting film have been combined to fabricate the superconducting weak-link element. Such a weak-link element is utilized as a magnetic quantum flux detector capable of detecting a feeble magnetic field or as a microwave/millimeter-wave detector of high sensitivity. The magnetic quantum flux detector has as high a flux resolving power as 10.sup.-9 Oe., and is applied to a magnetoencephalogram detector and a magnetocardiogram detector. The microwave detection range of the weak-link element can cover a high frequency band up to 10.sup.12 Hz which cannot be attained with another semiconductor element. In this manner, the superconducting device furnished with the superconducting weak-link element exhibits the excellent performance as the detector for the electromagnetic waves. Since, however, the Nb-type superconducting material in the prior art has a critical temperature of 23 K. or below, also the superconducting device formed using the Nb-type superconducting material has inevitably been operated in the liquid helium (4.2 K.).
Such a known example is stated in "IEEE Transactions on Magnetics," vol. MAG-21, No. 2, MARCH 1985, pp. 932-934.